Mitigation of hydrogen cyanide in aerogels

ABSTRACT

An embodiment of the present invention describes aerogel materials comprising an additive comprising a compound comprising at least two different metal elements. Another embodiment, involves aerogel particulates in combination with said compound. Said compound preferably comprises at least two different transition metal elements and may be in an oxide form.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This invention was partially made with Government support under ContractN65540-04-C-0008 awarded by the United States Navy. The Government mayhave certain rights in parts of this invention.

FIELD OF THE INVENTION

This invention relates in general to methods of reducing hydrogencyanide (HCN) concentrations in aerogels and aerogel composites.

SUMMARY OF THE INVENTION

Embodiments of the present invention describe an aerogels with reducedhydrogen cyanide (HCN) emission and a method for preparing the same.Accordingly, a method for preparing an aerogel (and aerogel composite)comprises the steps of: providing a solution comprising gel precursors;incorporating an additive into said solution prior to gellation thereof,said additive comprising a compound comprising of at least two differentmetal elements; allowing said solution to form a gel; and drying saidgel thereby resulting in an aerogel material. Said compound may compriseat least two different transition metal elements. A further optionalstep involves introducing the solution into a fibrous material; In oneembodiment said compound is in oxide form. A non-limiting example ofsuch oxides includes those with a general chemical formula of AB₂O₄where A and B represent transition metal elements. Such compounds may befurther exemplified by spinels. In another embodiment the additivecomprises a mixture of metal oxides. In a specific embodiment, thecompound comprises iron, copper, manganese, molybdenum or a combinationthereof. In yet another embodiment, the compound has a general formula:(Fe,Mn)(Fe,Mn)₂O₄:CuO. The additive may be present at between about 0.1%and 10% by weight of the aerogel. In one embodiment, the aerogel isbased on silica, titania, zirconia, alumina, hafnia, yttria, ceria,carbides nitrides or a combination thereof. In another embodiment, thefibrous material (of an aerogel composite) is in the form of a felt,mat, batting, chopped fibers or a combination thereof. Anotherembodiment describes a composition comprising aerogel particulates and acompound, said compound comprising at least two different metals as inthe previous embodiments.

DESCRIPTION

Embodiments of the present invention concern methods for reducinghydrogen cyanide concentration in aerogel materials. A specific exampleinvolves aerogel composites that can be utilized for firewall barrierapplications wherein the aerogel matrix is reinforced with anitrogen-containing carbon fiber felt. In some instances, thenitrogen-containing carbon fiber is based on oxidized polyacrylonitrilewhich has excellent flame resistance. However, it is well known thatpolyacrylonitrile-based carbon fibers contain nitrogen and can releasesignificant amounts of hydrogen cyanide (e.g. 30-50 ppm) during flameexposure. For certain applications, it is required that the HCN emissionduring flaming exposure be less than 30 ppm, thereby requiring reductionin HCN gas emitted. In this case, there are not many options fordirectly reducing the HCN emission from the carbon fiber other than tomodify the chemical composition of the carbon fiber by, for example,heat treatment, coating, or chemical doping.

It is taught in U.S. Pat. No. 4,115,353 that doping alkali metalcarbonates and cupric oxide into a styrene/acrylonitrile copolymer isbeneficial in reducing the amounts of HCN emission on combustion of thecopolymer. Therefore a change in the chemical composition of the sourceof HCN is required.

U.S. Pat. No. 5,720,785 discloses a method for reducing the hydrogencyanide and ammonia content from a nitrogen-containing coal feed duringcombustion by mixing an iron-containing compound with the coal feed. Inorder for this method to effectively reduce the HCN concentration, thecoal feed must be pulverized into fine particles so that theiron-containing compound can be mixed with the coal feed. Obviously,this method is not suitable for systems in which the HCN-emittingcomponent has a fibrous structure. In addition, this method requiresexceedingly high temperatures (over 1480° C.) and pressures (greaterthan 300 psig) in certain reaction zones. Such temperatures andpressures are not practical for most fire resistance applications.

Some previous efforts have taught methods to catalytically oxidize HCNin exhaust gases wherein the HCN is substantially released from theHCN-emitting compound into an exhaust gas stream and subsequentlymitigated. These methods are not suitable for the applications ofconcern here in that if the generated HCN is allowed to enter theatmosphere, it will be very difficult to mitigate under the environmentsof real fire resistance applications. Therefore, a need still exists foran effective and practical system for mitigation of HCN in aerogelsmaterials.

Aerogel materials are excellent insulators due mainly to their lowdensity and highly porous structure. The sol-gel process is one methodfor preparing gel materials; where a solution (sol) comprisingprecursors for the gel materials is prepared and upon gellation andsubsequent drying results in an aerogel material. Sol-gel process isdescribed in detail in Brinker C. J., and Scherer G. W., Sol-GelScience; New York: Academic Press, 1990; hereby incorporated byreference.

Within the context of embodiments of the present invention “aerogels” or“aerogel materials” along with their respective singular forms, refer togels containing air as a dispersion medium in a broad sense, and includeaerogels, xerogels and cryogels in a narrow sense. The chemicalcomposition of aerogels can be inorganic, organic (including polymers)or hybrid organic-inorganic. Examples of inorganic aerogels include, butare not limited to silica, titania, zirconia, alumina, hafnia, yttria,ceria, carbides and nitrides. Organic aerogels can be based on compoundssuch as but are not limited to: urethanes, resorcinol formaldehydes,polyimide, polyacrylates, chitosan, polymethylmethacrylate, members ofthe acrylate family of oligomers, trialkoxysilyl terminatedpolydimethylsiloxane, polyoxyalkylene, polyurethane, polybutadiane,melamine-formaldehyde, phenol-furfural, a member of the polyether familyof materials or combinations thereof. Examples of organic-inorganichybrid aerogels include, but are not limited to: silica-PMMA,silica-chitosan, silica-polyether or possibly a combination of theaforementioned organic and inorganic compounds. Published US patentapplications 2005/0192367 and 2005/0192366 teach extensively of suchhybrid organic-inorganic materials and are hereby incorporated byreference in their entirety.

Drying may be accomplished using a variety of methods known in the art.U.S. Pat. No. 6,670,402 herein incorporated by reference, teaches dryingvia rapid solvent exchange of solvent(s) inside wet gels usingsupercritical CO₂ by injecting supercritical, rather than liquid, CO₂into an extractor that has been pre-heated and pre-pressurized tosubstantially supercritical conditions or above to produce aerogels.U.S. Pat. No. 5,962,539 herein incorporated by reference, describes aprocess for obtaining an aerogel from a polymeric material that is inthe form a sol-gel in an organic solvent, by exchanging the organicsolvent for a fluid having a critical temperature below a temperature ofpolymer decomposition, and supercritically drying the fluid/sol-gel.U.S. Pat. No. 6,315,971 herein incorporated by reference, disclosesprocesses for producing gel compositions comprising: drying a wet gelcomprising gel solids and a drying agent to remove the drying agentunder drying conditions sufficient to minimize shrinkage of the gelduring drying. Also, U.S. Pat. No. 5,420,168 herein incorporated byreference describes a process whereby Resorcinol/Formaldehyde aerogelscan be manufactured using a simple air drying procedure. Finally, U.S.Pat. No. 5,565,142 herein incorporated by reference describessubcritical drying techniques. The embodiments of the present inventioncan be practiced with drying using any of the above techniques. In someembodiments, it is preferred that the drying is performed at vacuum tobelow super-critical pressures (pressures below the critical pressure ofthe fluid present in the gel at some point) and optionally using surfacemodifying agents. Aerogels can be opacified with compounds such as butnot limited to: B₄C, Diatomite, Manganese ferrite, MnO, NiO, SnO, Ag₂O,Bi₂O₃, TiC, WC, carbon black, titanium oxide, iron titanium oxide,zirconium silicate, zirconium oxide, iron (I) oxide, iron (III) oxide,manganese dioxide, iron titanium oxide (ilmenite), chromium oxide,silicon carbide or mixtures thereof.

Nitrogen-containing hydrocarbons may form toxic HCN during degradationin high temperatures events such as fires. A heat flux of about 40 kW/m²has been associated with that arising from typical fires (Behavior ofCharring Solids under Fire-Level Heat Fluxes; Milosavljevic, I.,Suuberg, E. M.; NISTIR 5499; September 1994). However, generallyspeaking, temperatures high enough to initiate HCN formation fromnitrogen-containing hydrocarbons are of concern here.

Aerogels comprising an inorganic composition are excellent materials forfire barriers, though there may be instances where organic or hybridorganic-inorganic aerogels are used. Furthermore, they can be preparedin flexible form for example when reinforced with fibrous materials. Thefibrous materials may comprise organic polymer-based fibers (e.g.polyethylenes, polypropylenes, polyacrylonitriles, polyamids, aramids,polyesters etc.) inorganic fibers (e.g. carbon, quartz, glass, etc.) orboth and in forms of, wovens, non-wovens, mats, felts, battings, loftybattings, chopped fibers, or a combination thereof. Aerogel compositesreinforced with a fibrous batting, herein referred to as “blankets”, areparticularly useful for applications requiring flexibility since theycan conform to three-dimensional surfaces and provide very low thermalconductivity. Aerogel blankets and similar fiber-reinforced aerogelcomposites are described in published US patent application2002/0094426A1 and U.S. Pat. Nos. 6,068,882, 5,789,075, 5,306,555,6,887,563, and 6,080,475, all hereby incorporated by reference, in theirentirety. Some embodiments of the present invention utilize aerogelblankets, though similar aerogel composites (e.g. those disclosed byreference) may also be utilized.

Aerogels reinforced with nitrogen-containing hydrocarbon-based fibersare of particular interest. Although the fibers may release HCN duringhigh temperature events, the HCN may be consumed if there is sufficientoxygen in the area and if the temperature is sufficiently high, makingfurther oxidation of HCN possible. It should be noted that althoughfibers may constitute the source of HCN is these aerogel materials,other components of an aerogel composite may also be responsible for thesame. For instance particulates as additives or reinforcement phases mayalso comprise nitrogen-containing hydrocarbons and thus can liberate HCNin high temperature events.

Embodiments of the present invention describe aerogels materials withadditives for mitigation of HCN. The aerogel matrix can serve as anideal supporting matrix for the additional additives that catalyze HCNoxidation. These additives can be finely dispersed with their reactivesurface sites exposed to the channels in the highly porous aerogelmatrix. Thus, the existence of the aerogel architecture makes HCNmitigation with the above approach possible.

According to embodiments of the present invention, additives comprisinga compound comprising at least two different metal elements, are used tocatalyze HCN oxidation reactions in an aerogel (or aerogel composite)material. The metal elements are preferably transition metals. In oneembodiment, the compound is in oxide form. A non-limiting example ofsuch oxides includes those with a general chemical formula of AB₂O₄where A and B represent transition metal elements. Such compounds may befurther exemplified by spinels.

In another embodiment the additive comprises a mixture of metal oxides.For instance oxides comprising one or more metal elements can becombined in various ratios. Transition metal elements are preferred;however, a mixture of transition metal and non-transition metal oxidesmay also be desired.

In a specific embodiment, the compound comprises iron, copper,manganese, molybdenum or a combination thereof. In yet anotherembodiment, the compound has a general formula: (Fe,Mn)(Fe,Mn)₂O₄:CuO.Dispersion of these additives as fine particles (e.g. nano-sized) cansignificantly reduce HCN concentration therein (see table 1.) Forinstance, powder forms of inorganic pigments belonging to the C.I.Pigment Black 26 chemical family are used as the additives. A generalchemical formula of this family is (Fe,Mn)(Fe,Mn)₂O₄:CuO (ManganeseFerrite Black Spinel) where an example of such compounds is commerciallyavailable from Ferro Corporation under the trade name of F-6331-2 COALBLACK®. Typically these additives are powders with mean particle size ofless than about 100 nanometers.

The amount of the additive used can be from 0.1 to 10 percent by weightbut is preferably between 0.5 and 2.5 percent based on the aerogelweight. About 1 percent by weight based on the aerogel weight is mostpreferred.

Incorporation of the additive into the aerogel structure can be carriedout during synthesis of the gel. One route is via dispersion of theadditive powder in the sol comprising the aerogel precursors. It isnoted that the additive may be incorporated into the sol at any pointbefore it gels. Gel formation may be understood as the point where thesol polymerizes into a continuous network and/or displays thecharacteristic of increased resistance to flowing. In some instances itmay be desired to utilize a dispersant (surfactant) to facilitatedispersion of the additive.

In a non-limiting example, an amount of DISPERBYK-184® (manufactured byBYK-Chemie Corporation) is added to the sol. Satisfactory dispersion isgenerally achieved by vigorously stirring the mixture of sol, additivepowder, and dispersant for over 30 minutes. Subsequent to the mixing, agelling agent may be introduced to the sol so that it gels in less time(e.g. 5 minutes.) Immediately after the introduction of the gellingagent, the sol can be cast onto a fibrous matrix such as a carbon felt,allowed to gel, aged, and dried in supercritical CO₂ to yield a carbonfiber/aerogel composite with finely dispersed HCN mitigating additivesinside the aerogel matrix.

The effectiveness of the additives in mitigating HCN emission fromaerogel composites during flaming exposure is demonstrated by thefollowing example:

In this example, the HCN emission levels of composite samples containingdifferent amounts of F-6331-2 COAL BLACK® under flaming exposure weremeasured. The samples are silica aerogel composites reinforced withpolyester fibers. The test was carried out in general accordance withASTM E 662, and was supplemented with gas analysis using FTIRspectroscopy. A summary of the results is shown in table 1.

Additive % (based on Concentration % Reduction Sample ID weight ofcomposite) of HCN (ppm) in HCN 1 0 34 0 2 0.5 8 76 3 1 8 76 4 2.5 0 1005 5 0 100

Another embodiment involves a composition comprising aerogelsparticulates and additives, wherein said additives comprise of acompound comprising at least two different metals. The metal elementsare preferably transition metals. Furthermore, the compound may be inoxide form. A non-limiting example of such oxides includes those with ageneral chemical formula of AB₂O₄ where A and B represent transitionmetal elements. Such compounds may be further exemplified by spinels. Inanother aspect, the additives comprise a mixture of metal oxides. Forinstance oxides comprising one or more metal elements can be combined invarious ratios. Transition metal elements are preferred; however, amixture of transition metal and non-transition metal oxides may also bedesired.

The aerogel particulates as used herein include beads, substantiallyspherical particles, irregular particles and the like. Furthermore,aerogel beads of various diameters may be utilized. Generally, beads ofabout 0.1 μm in diameter or larger can be used. In one instance, beadswith diameters larger than about 0.5 mm or larger than about 1.0 mm areused. In some instances, the beads can have diameters as large as about2 cm.

The aerogel particles can be bound to each other and/or the additiveswith a binder. Both aqueous and non-aqueous (organic) based binders maybe suitable depending on the nature of the aerogel particulates and theend application of the composition. A non-limiting list of binderscomprises: acrylics, silicones, phenolic polymers, acrylic styrenecopolymers, vinyl acrylic polymers, epoxy acrylic copolymers, acrylicvinyl acetate copolymers, styrene butadiene copolymer and celluloseacetate copolymers. US2002/0025427 and US2003/0003284 describe severalcommercially available polymeric binders such as Mowilith® or Mowital®also applicable to the present embodiment. The following disclosuresWO03064025, WO2003/097227, WO2003/0215640, US2004/0077738, US2005/025952all of which are incorporated by reference pertain to insulation panelsthat can be formed from a composite comprising hydrophobic silicaaerogel particles and an aqueous binder.

It is anticipated that many minor modifications and variations could bemade in the invention described herein without changing the essentialelements thereof and it is understood that all such modifications andvariations are embraced within the purview of this invention.

1. A method of incorporating an additive into an aerogel materialcomprising the steps of: providing a solution comprising gel precursors;incorporating an additive into said solution prior to gellation thereof,said additive comprising a compound having a general formula(Fe,Mn)(Fe,Mn)₂O₄:CuO; mixing a nitrogen containing fibrous materialwith said solution; allowing said solution to form a gel; and dryingsaid gel thereby resulting in an aerogel material.
 2. The method ofclaim 1 wherein said compound comprises C.I. pigment black
 26. 3. Amethod of incorporating an additive into an aerogel material comprisingthe steps of: providing a solution comprising gel precursors;incorporating an additive into said solution prior to gellation thereof,said additive comprising F-6331-2 COAL BLACK; mixing a nitrogencontaining fibrous material with said solution; allowing said solutionto form a gel; and drying said gel thereby resulting in an aerogelmaterial.
 4. The method of claim 1 wherein the additive comprises amixture of metal oxides.
 5. The method of claim 1 wherein said compoundhas a spinel structure.
 6. The method of claim 2 wherein said additivecomprises iron, copper, manganese, molybdenum or a combination thereof.7. The method of claim 1 wherein the additive is present at betweenabout 0.1% and about 10% by weight of the aerogel composite.
 8. Themethod of claim 1 wherein the gel is dried with a supercritical fluid.9. The method of claim 1 wherein the aerogel comprises an inorganicmaterial.
 10. The method of claim 9 wherein the aerogel comprisessilica, titania, zirconia, alumina, hafnia, yttria, ceria, carbidesnitrides or a combination thereof.
 11. The method of claim 1 furthercomprising the step of introducing said solution into a fibrousmaterial.
 12. The method of claim 11 wherein the fibrous material is afelt, mat, batting, chopped fibers or a combination thereof.
 13. Anaerogel material comprising: at least one nitrogen containing fibrousmaterial; at least one additive dispersed within said aerogel material,wherein said additive comprises a compound having a general formula(Fe,Mn)(Fe,Mn)₂O₄:CuO.
 14. The aerogel material of claim 13 wherein saidcompound comprises C.I. pigment black
 26. 15. The aerogel material ofclaim 13 wherein said additive comprises an oxide.
 16. The aerogelmaterial of claim 13 wherein the additive comprises a mixture of metaloxides.
 17. An aerogel material comprising: at least one nitrogencontaining fibrous material; at least one additive dispersed within saidaerogel material, wherein said additive comprises F-6331-2 COAL BLACK.18. The aerogel material of claim 13 wherein said compound has a spinelstructure.
 19. The aerogel material of claim 13 wherein said additivecomprises iron, copper, manganese, molybdenum or a combination thereof.20. The aerogel material of claim 13 wherein the additive is present atbetween about 0.1% and 10% by weight of the aerogel material.
 21. Theaerogel material of claim 13 wherein the aerogel comprises an inorganicmaterial.
 22. The aerogel material of claim 13 wherein the aerogelstructure comprises silica, titania, zirconia, alumina, hafnia, yttria,ceria, carbides nitrides or a combination thereof.
 23. The aerogelmaterial of claim 13 wherein the fibrous material is in the form of afelt, mat, batting, chopped fibers or a combination thereof.
 24. Acomposition comprising aerogel particulates, a nitrogen containingfibrous material and an additive comprising a compound, said compoundcomprising at least two different metals and cupric oxide.
 25. Thecomposition of claim 24 wherein said compound is an oxide.
 26. Thecomposition of claim 24 wherein the additive comprises a mixture ofmetal oxides.
 27. The composition of claim 24 wherein said compoundcomprises C.I. pigment black
 26. 28. The composition of claim 24 whereinsaid compound has a spinel structure.
 29. The composition of claim 24wherein said compound comprises iron, copper, manganese, molybdenum or acombination thereof.
 30. The composition of claim 24 wherein saidcompound has a general chemical formula of (Fe,Mn)(Fe,Mn)₂O₄:CuO. 31.The composition of claim 24 wherein the additive is present at betweenabout 0.1% and 10% by weight of the aerogel material.
 32. Thecomposition of claim 24 wherein the aerogel comprises an inorganicmaterial.
 33. The composition of claim 24 wherein the aerogel structurecomprises silica, titania, zirconia, alumina, hafnia, yttria, ceria,carbides nitrides or a combination thereof.
 34. The composition of claim24 wherein the fibrous material is in the form of a felt, mat, batting,chopped fibers or a combination thereof.